The more poetic translations are "Swift death to evil" and "Slay evil immediately."

Aku = Evil
Soku = Immediate (more emphasis on "immediate" as opposed to "swiftly" or "quickly")
Zan = Slay (Depending on how you see it; literally it means "chop" but this changes slightly in context. Here it probably means "slay".
It is a motto that united soldiers in late 19th century Japan, during the period of the Meiji Revolution.

Though some may consider such a "law" an extreme measure, those who followed it did so religiously. Believing that no matter what side they fought for, that as long as they upheld Aku Soku Zan, they would accomplish their mission.
by Mytheos-ASB February 2, 2007
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The code of Hajime Saitou of the Shinsingumi. This may or may not apply to all members of the Shinsingumi. It is at least what many strived to do.
by Rayo September 25, 2003
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Sin. Swift. Slay. Often translated as kill evil swiftly.
by WannaBeSaitoh'sGirl August 6, 2003
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used by the captain of the first shinsengumi unit saito hajime meaning swift death to evil
cant think of anything to put that contains the words aku soku zan
by fayte October 16, 2006
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